Touch panels have been a focus of attention as an input device in operating computer systems and the like. In a capacitance touch panel for example, a position of a target object such as an operator's finger or a pen is detected on the basis of electrostatic capacitance changes between a driving line and a sensing line. Conventionally, such a touch panel as described is placed on a display panel provided by a liquid crystal display panel for example. The touch panel provided on the display panel as described above is called “out-cell touch panel” for example.
However, out-cell touch panels have problems that they increase overall weight and thickness of the display panel and touch panel, and that they increase power demand for driving the touch panel. Efforts have been made in recent years therefore, in developing a touch-sensor-embedded display panel which is a display panel incorporating a touch panel (or in more precise wording, a display panel with built-in touch sensor functions for a touch panel). A touch panel in such a touch-sensor-embedded display panel as described is called “in-cell touch panel” for example. Touch-sensor-embedded display panels make it possible to decrease overall weight and thickness of the display panel and touch panel, and also decrease drive power for the display panel and touch panel as a whole.
FIG. 18 is an equivalent-circuit diagram of a driving pixel 321 and a sensing pixel 322 in a touch-sensor-embedded liquid crystal display panel disclosed in Patent Document 1. Herein, the driving pixel 321 and the sensing pixel 322 are pixels corresponding to a driving line and a sensing line respectively. The driving pixel 321 is constituted by a transistor T1, a pixel electrode Epix1, two common electrodes COM1, a liquid crystal capacitance Clc1 formed between the pixel electrode Epix1 and one of the common electrodes COM1, and an auxiliary capacitance Cst1 formed between the pixel electrode Epix1 and the other of the common electrodes COM1. As depicted, the driving pixel 321 includes the liquid crystal capacitance Clc1 and the auxiliary capacitance Cst1 as capacitances for holding a voltage which corresponds to a signal (source signal) applied to a source line SL1. The sensing pixel 322 is constituted by a transistor T2, a pixel electrode Epix2, two common electrodes COM2, a liquid crystal capacitance Clc2 formed between the pixel electrode Epix2 and one of the common electrodes COM2, and an auxiliary capacitance Cst2 formed between the pixel electrode Epix2 and the other of the common electrodes COM2. As depicted, the sensing pixel 322 includes the liquid crystal capacitance Clc2 and the auxiliary capacitance Cst2 as capacitances for holding a voltage which corresponds to a signal (source signal) applied to a source line SL2. In FIG. 18, Cf represents a capacitance (hereinafter called “detection capacitance”) that changes its capacitance value when approached by a finger tip, pen or the like; whereas Cp1 through Cp4 represents parasitic capacitances.
If this liquid crystal display panel is not an IPS (In-Plane Switching) type (hereinafter called “non-IPS type”), but is an VA (Vertical Alignment) type for example, one of the two common electrodes COM1 is disposed on a CF (Color filter) substrate while the other is disposed on a TFT (Thin Film Transistor) substrate. Likewise, one of the two common electrodes COM2 is disposed on the CF substrate while the other is disposed on the TFT substrate. The common electrodes COM1 and COM2 in the touch-sensor-embedded liquid crystal display panel function as common electrodes in a liquid crystal display device during display operation, whereas they function as a driving line and a sensing line respectively during touch detection operation. In the touch detection operation, the common electrodes COM1, COM2 are driven independently per predetermined region. In the present Description, “display operation” refers to an operation performed for displaying images in a display panel, whereas “touch detection operation” refers to an operation performed for detecting coordinates of a touch position of a finger, for example, in the display panel.